1. Field of the Invention
The present invention relates to a semiconductor device using an SOI (Silicon On Insulator) substrate. More particularly, it relates to a high speed bipolar semiconductor device, and a drive circuit made up of the semiconductor devices.
2. Description of the Related Art
A bipolar transistor will be described as a prior-art example of a semiconductor device of an SOI structure. An example of this kind of bipolar transistor is disclosed in xe2x80x9cA 6-xcexcm2 bipolar transistor using 0.25-xcexcm process technology for high-speed applicationsxe2x80x9d (Bipolar/BiCMOS Circuits and Technology Meeting 1998 proceedings).
FIG. 2 shows a cross sectional view of the bipolar transistor described in the prior-art example.
In FIG. 2, a reference numeral 1 denotes a silicon substrate, 2 a buried silicon dioxide, 3 a silicon layer, 4 an n+ buried layer, 5 an nxe2x88x92 silicon epitaxial layer, 6 a field oxide, 7 an n+ collector contact layer, 11 an isolation oxide, 14 a p+ extrinsic base polysilicon electrode, 15, 18, and 22 silicon dioxides, 16 an intrinsic base layer, 17 a graft base, 19 an n+ polysilicon sidewall, 20 an n+ polysilicon emitter, 21 a single crystal silicon emitter, and 23, 24, 25 metal electrodes.
Further, as a second prior-art example, a part of the configuration disclosed in JP-A No. 86298/1995 is shown in FIG. 3. Some reference numerals are used in common with FIG. 2, and as other reference numerals, 12 denotes a polysilicon, and 30 a silicon dioxide.
In general, a semiconductor device generates heat during operation. Particularly, a bipolar transistor generates a large amount of heat, and radiates the heat to a silicon substrate. In recent years, an SOI (Silicon On Insulator) substrate with an insulating film buried therein has been used in place of a silicon bulk substrate aiming at reducing the substrate capacitance and merging complementary MOS therewith. The prior-art example as shown in FIG. 2 is a preferable example thereof. The thermal conductivity of the buried silicon dioxide 2 is about {fraction (1/100)} as compared with silicon. Therefore, the generated heat is less likely to be radiated to the substrate as compared with the silicon bulk substrate, resulting in an increase in temperature in the transistor. Consequently, the change in the electrical characteristics of the transistor becomes remarkable.
Particularly, a hetero bipolar transistor using silicon-germanium as a base is reduced in current gain with an increase in temperature. This indicates the phenomenon that the electrical characteristics of the transistor differ according to the difference in density of the transistor arrangement, and the conditions of use. For this reason, a problem occurs that it is impossible to attain the performances as designed in a designed circuit. Further, the transistor is required to be used at high current for high-speed operation. Accordingly, the high-speed operation of the transistor is restricted in order to prevent the heat generation due to a current. Such a phenomenon holds true for not only the hetero bipolar transistor, but also semiconductor devices of an SOI structure such as a bipolar transistor using an SOI, a high-power MOS field effect transistor of an SOI structure, and a resistor used at a high current density.
As a means for solving the problem of heat radiation, for the bipolar transistor disclosed in the second prior-art example shown in FIG. 3, an isolation oxide 11 and a polysilicon 12 are buried in an isolation groove penetrating from a silicon substrate 1 to a buried silicon dioxide 2 and a silicon layer 3. As a result, the generated heat flows through the isolation oxide 11 to the polysilicon 12, and is radiated to the silicon substrate 1. In this case, the isolation oxide 11 is required to be reduced in film thickness in order to enhance the effect of heat radiation. However, if the isolation oxide 11 is reduced in thickness, the capacitance arising at the isolation groove portion of the substrate capacitance increases. Incidentally, the structure of the second prior-art example is a part of the one described in the JP-A No. 86298/1995. With this structure, heat radiation is carried out not through the silicon substrate, but at a heat radiation electrode through the groove in which a polysilicon is buried.
It is generally an object of the present invention to enhance the effect of heat radiation in a semiconductor device. In particular, it is an object to provide a bipolar semiconductor device which has been improved in effect of heat radiation without increasing the substrate capacitance, and operates at high speed. Further, it is another object of the present invention to provide an optical transmission system using the semiconductor devices.
The general outline of the present invention disclosed by this application will be described briefly below. Namely, a semiconductor device in accordance with the present invention is characterized in that the heat generated within the semiconductor device is led by a single crystal silicon or polysilicon layer, and radiated from the single crystal silicon or polysilicon to the silicon substrate through another single crystal silicon or polysilicon connected to the silicon substrate.
Below, a typical example of the present invention will be described briefly.
A bipolar transistor is characterized by having such a structure that an extrinsic single crystal silicon, or polysilicon base electrode is in contact with a single crystal silicon or polysilicon buried in an isolation groove, or an insulating film sufficiently thinner than the thickness of a field oxide is inserted in the interface therebetween. As a result, the heat generated in an intrinsic area passes through the extrinsic single crystal silicon or polysilicon base electrode having a thermal conductivity which is about 100 times that of a silicon dioxide, and radiated through the single crystal silicon or polysilicon in the isolation groove to the silicon substrate.
If this effect is combined with the effect of heat radiation through an insulating film on the inner sidewall of the isolation groove in the prior art, when the area of the emitter is about 0.2xc3x971 xcexcm2, and the thickness of the silicon dioxide on the inner sidewall of the isolation groove is 0.2 xcexcm, heat radiation is two times or more effective than the prior-art example. The effect becomes noticeable for a transistor formed with a more advanced submicron patterning. In other words, the effect becomes noticeable due to the following fact: the distance between the intrinsic area and the isolation groove is reduced, resulting in a decreased thermal resistance at the extrinsic base electrode. Further, since the surface area of the sidewall of the isolation groove is reduced, the thermal conductivity of heat passing through the sidewall is reduced. As a result, the thermal resistance at this portion is increased. Accordingly, the effect of heat radiation due to the structure of this application is further improved.
By reducing the thickness of the silicon dioxide buried in the isolation groove, it is also possible to improve the effect of heat radiation with the prior-art structure. However, when the thickness is reduced, the substrate capacitance arising through the silicon dioxide between the collector area and the silicon substrate is increased. On the other hand, with the structure of this application, silicon in the isolation groove is used as the path for heat radiation. Therefore, even if the thickness of the silicon dioxide buried in the isolation groove is increased, the reduction in the effect of heat radiation is xc2xd or less as compared with the prior-art structure. Accordingly, it is possible to sufficiently ensure the effect of heat radiation while inhibiting the increase in the substrate capacitance.
When the extrinsic base electrode and the silicon in the isolation groove are in direct contact with each other, the impurities in the extrinsic base electrode diffuse into the silicon in the isolation groove. However, the diffusion depth of the impurities due to the thermal budget in a normal high-performance bipolar transistor formation process is about 0.2 xcexcm even when a polysilicon is used, and thus it will not be deeper than the field oxide thickness. When a single crystal silicon is used, the diffusion depth is shallower as compared with the polysilicon. The amount of increase in base-collector parasitic capacitance arising due to the diffusion of impurities is about 3% of the total base-collector parasitic capacitance, which is a negligible value.
If a consideration is given to the insulating property between the extrinsic base electrode and the silicon substrate, the resistance value of the silicon in the isolation groove is required to be 1 Mxcexa9 or more. In general, the resistivity is 104 xcexa9xc2x7cm or more for an undoped polysilicon. Whereas, it may be 103 xcexa9xc2x7cm or more even for a single crystal silicon. In this case, the resistance value between the extrinsic base electrode and the silicon substrate exhibits 1 Mxcexa9 or more even when a single crystal silicon is used. Therefore, the resistivity is required to be 103 xcexa9xc2x7cm or more from the viewpoint of the insulating property of this portion in either case where the silicon in the isolation groove is single crystalline or polycrystalline.
When the thermal processing in the transistor formation step is more often performed than normal, the impurities in the extrinsic base electrode diffusing into the silicon in the isolation groove increase in amount. In such a case, a thin insulating film becomes required to be inserted between the extrinsic base electrode and the silicon in the isolation groove. In this case, the effect of heat radiation varies according to the thickness of the insulating film. If the insulating film thickness is 10 nm, the effect of heat radiation is reduced by as much as twice that of a prior-art structure.
The current density passing through a transistor is expected to increase up to twice thereof in the future, and hence the effect of heat radiation is required to be twice the current effect of heat radiation. Therefore, it is considered desirable that the insulating film thickness is 10 nm or less.
For an MOS field effect transistor, heat is radiated from a single crystal silicon or polysilicon layer formed on a source or drain to the single crystal silicon or polysilicon in the isolation groove.
For a resistance element formed with a polysilicon, it is also possible to radiate heat directly from the resistor to the silicon in the isolation groove.
The same effects as illustrated in the description on the bipolar transistor are also exerted in the cases of the MOS field effect transistor and the resistor.
Other and further objects, features and advantages of the invention will appear more fully from the following description.